organisms include marine algae, fillimentous Bryozoa, coelenterate, tuni- 

 cates and calcareous and siliceous sponges. Semi-motile fouling organisms 

 are those which possess the power to move if not restricted by outer 

 forces, such as the growth of other organisms surrounding it. This group 

 includes sea anemones, worms, certain Crustacea and mollusks . Motile 

 organisms such as worms, certain mollusks such as sea slugs and snails, 

 may affect the corrosion of metal directly or indirectly due to the slimy 

 film secreted by them (Clapp, in Uhlig, 19^8). Ways in which fouling or- 

 ganisms affect the corrosion of steel piling are given in paragraph 7, 

 "Marine Organisms" of Section V. 



8. Bacteria 



Seawater contains numerous types of bacteria. Anaerobic bac- 

 teria which thrive in oxygen-free environments where sulfate is present 

 are of concern to corrosion engineers where stagnant water exists. 

 Sulfate reducing bacteria can cause corrosion of material without the 

 presence of oxygen. 



A factor of significance in corrosion protection is that many ma- 

 terials are suited to the metabolism of some types cf marine bacteria 

 (Muroaka, I963). For this reason, many of the protective coatings for 

 steel may be damaged or destroyed by marine bacteria. Muroaka gives 

 four ways in which bacteria takes part in the fouling of marine struc- 

 tures as follows : 



a. Being a source of food for barnacles. 



b. Affording footholds for other animals. 



c. Aiding sessile organisms in depositing 

 their calcareous cements. 



d. Discoloring glazed or bright surfaces (fouling 

 proceeds faster on dull dark surfaces). 



Section IV. CORROSION OF STEEL PILING IN SEAWATER 



1. General 



The corrosion of steel piling in seawater is caused by electro- 

 chemical action. This corrosion process primarily involves the steel,, 

 an electrolyte, in this case seawater, and oxygen. As a result of the 

 corrosion process, iron, the principal constituent of steel, is restored 

 to the state in which it is mined as ore (iron oxide). Iron oxides that 

 form on the surface of steel as a result of the corrosion process may 

 greatly retard further reaction. Black iron oxide (Fe30U) offers good 

 protection from further corrosion whereas the iron oxides (Fe203 and 

 Fe203. X H2 ) do not appreciably protect the underlying metals (Baxter 

 and Steiner, I960). 



